I have not benchmarked it. C triggers are probably always going to outperform anything else, not least because the language handlers for other languages need to perform some glue operations. My instict would be that perl might perform better than PL/PGSQL, and it might not, but in any case is still likely to be slower than C.

That said, perl performance is adequate for many purposes. And even if you find you eventually need to write your triggers in C, it might work well to prototype in perl.